Science and Issues

Climate Moderator, Water as a

Water at the Earth's surface and in the atmosphere exerts a strong
moderating effect on climate. Generally speaking, the higher the water
content in the air, the more moderate (less extreme) the climate.

Prince Edward Island exemplifies a region with a maritime climate,
moderated by the presence of the Gulf of St. Lawrence. The mild
climate makes the area suitable for growing grain crops (shown here)
and potatoes.

Water Content in the Air

Proximity to water bodies causes humidity to be higher due to
evaporation of water from the water surface. All other things being
equal, the greater the distance from a major waterbody, the drier the
air.

The ability for air to hold water in the form of water vapor varies with
temperature. Hot air can have a very large water vapor content, while
cold air holds very little. One measure of the actual amount of water
vapor in the air is vapor pressure, which is a measure of the number of
water molecules in the air compared with the number of molecules of
other constituent gases in the air. Another term, saturation vapor
pressure, indicates the maximum amount of water vapor that the air could
hold; this is strictly a function of temperature. Relative humidity is
the percentage of the actual water vapor content compared with the
saturation vapor pressure. Saturated air has a relative humidity of 100
percent, whereas totally dry air (which never occurs in the atmosphere)
would have a relative humidity of 0 percent.

Orographic Lifting and Rain Shadows.

When air moves up and over a mountain or ridge, in a process called
orographic lifting, it cools as it rises. (As the pressure decreases,
the air "expands," and the air molecules move more
slowly). If the rising air cools enough, its temperature will reach the
dew point, the temperature at which air is saturated. At that
temperature, condensation will begin; this is the point at which water
vapor begins to be converted to liquid water, the result being clouds
and precipitation.

Because a steep mountainside can force air upward very quickly over a
short distance, orographic lift can produce heavy rain or snow along the
side of the mountain range that faces the wind, if enough moisture is
present in the air. In the western United States, some mountain ranges
are aligned north-to-south, with their western slopes facing the Pacific
Ocean and its abundant moisture. Ski resorts in the Cascade Range of
Washington and Oregon, and the Lake Tahoe region in the Sierra Nevada,
for example, benefit from orographically produced snowfall.

Moist air moving up the side of a mountain facing the prevailing
wind causes precipitation to fall in a process known as orographic
lifting. On the lee side, a rain shadow occurs, and precipitation is
sparse.

When the air rises to and passes over a mountain or ridge, it begins to
descend down the other side. Descending air warms, and as it does, the
saturation vapor pressure increases. But because no water is being added
to the air, the actual vapor pressure stays the same. Thus the relative
humidity becomes lower, and the air gets drier. This process causes the
lee side of the mountains (i.e., the side away from the prevailing
winds) to be a zone of limited precipitation known as a rain shadow.
Rain shadows are major contributors to the climate of certain parts of
the world. In the example above, the regions east of the Cascades in
Oregon and Washington, and east of the Sierra Nevada Mountains in
California, are
arid
as a result of the rain shadow effect.

Lake-Effect Snow.

Lake-effect snow is a localized, and sometimes heavy, snow that develops
downwind of large lakes. In the United States, it is most common south
and east of the Great Lakes, where residents experience the highest
annual snowfall totals anywhere east of the Rocky Mountains. A related
phenomenon, called sea-effect snow, occurs in coastal regions when cold
air spreads over warmer ocean water.

In these processes, the air passing over water picks up moisture, which
rises and forms clouds that produce snow. At least 80 kilometers (50
miles) of fetch (the length of lake over which air travels) is required
for a significant lake-effect snow. A greater temperature difference
between the air and water enhances the cloud-forming and snow-forming
process, as does steeply rising terrain downwind of a lake. Wind
direction also affects the snow intensity.

Although a lake-effect process may not always produce snow, the amount
of snow that can fall in these events can be enormous, reaching rates of
12.5 centimeters (5 inches) per hour. Parts of the Tug Hill Plateau in
upstate New York receive an average of 625 centimeters (250 inches) of
snow each winter, more than six times the typical amount in areas not in
lake-effect regions.

Moderation of Air Temperature

Water vapor is the main reason for the greenhouse effect, in which
certain gases in the atmosphere allow sunlight to pass through, but
absorb heat released from the Earth (when sunlight strikes the Earth it
changes from visible light to infrared radiation, or heat). Without this
effect the Earth would be about 33°C cooler than it is at present
(that is, 60°F cooler). Human-caused emissions, leading to
increased levels of carbon dioxide (CO
2
) and other gases in the atmosphere may accelerate the
greenhouse effect
.

Water vapor absorbs heat and releases it slowly. At night, when the
humidity is high, the atmosphere retains more heat, and nighttime
temperatures stay somewhat high. On dry nights, however, with little
water vapor to absorb heat, the atmosphere cools off rapidly.

Clouds act in much the same way as high humidity. The presence of clouds
means humid atmospheric conditions, and this promotes greater heat
retention, and higher nighttime temperatures, than on a clear (dry)
night.

Liquid water at the surface also can play a significant role in climate
moderation. Water takes much longer than air to heat up, and also longer
to cool, because it has much higher specific heat. Thus, on hot days,
water (oceans, lakes, and rivers) absorbs heat, keeping the air somewhat
cooler. When the air gets cool, however, water slowly releases heat to
the atmosphere, raising air temperatures. This is why temperatures along
coastlines are cooler in summer and warmer in winter relative to inland
areas (see side-bar). Temperature vary more the farther inland one
travels. There is some moderation of climate because condensation is an
exothermic process, and warms the atmosphere, particularly in wet
coastal areas (i.e., downwind from the ocean).

As an example of how distance from the ocean influences air temperature,
consider the states of Oregon and Washington. The Cascades Range divides
the two states into a western region, west of the mountains, and an
eastern region, east of the mountains. The climate of the western region
is moderated by the Pacific Ocean, and is mild and moist. But the
eastern region is relatively dry (the rainshadow effect) and warm, even
hot.

The coastal city of Astoria, Oregon experiences warmer daily minimum
temperatures and cooler maximum temperatures than Pendleton, Oregon
which is about 400 kilometers (250 miles) inland, and separated from the
ocean by the Cascades. Astoria's average monthly temperatures
also exhibit a smaller range: 5.5°C (41.9°F) in January, the
coolest month (compared to Pendleton's 0.8°C [33.5°F])
and 16°C (60.9°F) in the hottest month, August (compared to
Pendleton's 22.7°C [72.9°F] in July, its hottest
month).

Ironically, although higher humidities mean more moderate temperatures,
to human and animals they feel more extreme. A 32°C-day (90°F)
in Arizona feels hot, but the same temperature on a humid summer day in
Washington, D.C. feels stifling. Similarly, a cold, dry day feels less
extreme than a humid day at the same temperature. These effects, again,
are due to water's specific heat—its ability to conduct
heat (or cold) compared to dry air.

Internet Resources

SEA AND LAND BREEZES

A sea breeze occurs in the daytime along the shore of an ocean or large
lake when air over the sun-heated land becomes warmer than air over the
adjacent, relatively cool ocean. The heated air over land rises,
creating a localized low pressure zone into which the cooler sea air
moves (because air moves from higher to lower pressure). A sea breeze
helps keep daytime coastal temperature pleasantly mild, even though
inland areas may be hot.

At night, the air over the rapidly cooling land becomes cooler than the
air over the relatively warm ocean. Because the lower air pressure is
now over the ocean, the wind blows from land to water in what is known
as a land breeze.